The present invention relates generally to improvements in continuously variable transmissions and, more particularly but not by way of limitation, to continuously variable transmissions of the type having two pulleys, at least one of which is split into two axially positionable halves, mounted on parallel shafts and engaged by an endless belt.
It is well known that the efficiency of an internal combustion engine depends upon the speed at which the engine operates and this knowledge, coupled with the recognition that the earth's energy resources are finite, has led to a great deal of work directed toward the development of practical continuously variable transmissions that would enable various machines; for example, automobiles, to be operated at varying speeds using engines operating at their most efficient speeds. Should such transmissions come into widespread use, the resulting energy savings would have the effect of greatly extending the earth's reserves of fossil fuels such as petroleum and coal.
A type of continuously variable transmission that has attracted a great deal of attention from inventors and engineers employs axially split pulleys mounted on parallel input and output shafts and engaged by an endless belt to effect a variation in the relative rotation rates of the shafts. In this type of transmission, axial positioning of the halves of the pulleys adjusts the ratio of the radii at which the belt is engaged by the pulleys so that the rotation rate of the output shaft can be varied, by the adjustment of the pulleys, while maintaining a constant rotation rate at the input shaft.
While this type of continuously variable transmission has been used effectively in a number of specific applications, it has limitations which have prevented it from being placed in widespread use. In particular, power transmission via an endless belt has given rise to several problems which have not been solved prior to the present invention.
A major problem with transmissions of this type is that the belt may slip on the pulleys so that these transmissions are generally limited to low power applications. In part, this problem is associated with the economics of transmission construction. In order to maintain the cost of such transmissions at a level that makes them practical, it has been a common practice to adjust the spacing of the halves of only one pulley and spring load the halves of the other pulley so that the force exerted by a spring or springs determines both the grip of the pulleys on the belt and the setting of the ratio of the radii at which the belt is gripped by the pulleys; i.e., the effective "gear ratio" of the transmission. In practice, the spring forces limit the torque which can be transferred between the pulleys.
A second problem has been fatigue of the belt. During operation of the transmission, the belt is under tension on one side of a line between the centers of the shafts and relaxed on the other side of the line so that the belt is being continually flexed. This flexing causes heating that can, after a time, result in failure of the material of which the belt is constructed. Additional fatigue, and rapid belt wear, also arises from the bending of the belt and twisting of the belt against the pulleys at such times that the belt is engaged by the pulleys. While a partial solution to this problem can be achieved by using sectioned metal belts which deliver torque through compression, the cost of the belt then becomes a problem. Thus, the problem of belt fatigue remains largely unsolved.